1. Bring a large pot of water to a boil. Cut a cross on bottom of each tomato. Remove pot from heat, plunge tomatoes in water, and let sit for 1 to 2 minutes, until skins loosen at cross mark. Spoon tomatoes into a colander and rinse with cold water.
2. Skin and stem tomatoes; if really juicy, squeeze out some juice into sink. Crush pulp with your hands into a bowl, breaking it up into small chunks.
3. Pour oil into a large frying pan and heat over medium heat. Add garlic if using, tomato pulp, and a big pinch of salt. Cook 1 to 2 minutes, until pulp breaks down and releases juices. Using a slotted spoon, put pulp back in bowl, leaving juice in pan.
4. Simmer juice until thick enough to leave a mark when you scrape bottom of pan, 2 to 3 minutes. Check seasoning. If your tomatoes are not very tasty, add vinegar and a bit more salt, and chile flakes if you want a kick. Stir pulp and basil into sauce.
Make ahead: 1 week, chilled.
Note: Nutritional analysis is per serving.